1. Field of the Invention
The present invention relates to an image forming apparatus which controls recording conditions in accordance with any unevenness in the recording quality thereby forming an image of a uniform quality.
2. Description of the Related Art
Hitherto, various mechanical methods have been proposed for correcting any unevenness of recording quality. For instance, a method has been proposed in which the speed at which a recording paper is fed is varied in accordance with a change in the amount of eccentricity of the drive roller. In another known method, a roller is used which presses a paper feed belt and a drive roller to each other. These mechanical means, however, are still unsatisfactory in that they cannot eliminate unevenness of recording density to an acceptable level, due to a limit in the precision of mechanical processing and assembly of the parts.
Meanwhile, in the field of printing using a multi-nozzle ink jet head or a thermal head, it has been proposed to correct unevenness of printing quality attributable to fluctuation in the properties of printing heads by suitably controlling the level of image signals and/or printing conditions.
This proposed method, however, is effective only in the correction of unevenness of recording quality occurring in the direction of the main scan.
In some cases, however, unevenness regularly occurs on the recording medium due to various reasons such as eccentricity of a paper feed roller, expansion or contraction of a paper feed belt, oscillation of an optical system, e.g., a back scan by a light source of an image reader, caused by movements of parts around the recording region, and so forth. Such regular unevenness takes place also when the feeding speed of the recording paper is changed, e.g., when the leading end of a sheet of recording paper is caught in a fixing device while the central portion of the sheet is still under printing. Such regular unevenness in the recording quality is preferably eliminated by mechanical means. Actually, however, such mechanical correcting means inevitably causes unevenness in the sub-scan direction. Hitherto, no measures have been proposed for eliminating such unevenness of recording quality occurring in the sub-scan direction.